<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            padding: 0;
            margin: 0;
        }
        .box {
            width: 200px;
            height: 30px;
            margin: 10px auto;
        }
        .bottom {
            margin-left: 20px;
            margin-top: 5px;
            width: 100px;
            height: 30px;
            line-height: 30px;
            text-align: center;
            background-color: slateblue;
            display: none;
        }
    </style>
</head>
<body>
    <div class="box">
        <input type="checkbox">
        <span>两周内自动登陆</span>
        <div class="bottom">测试</div>        
    </div>
</body>
<script>
    var box = document.querySelector('.box')
    var bottom = document.querySelector('.bottom')
    box.onmouseover = function () {
        bottom.style = 'display:block'
    }
    box.onmouseout = function () {
        bottom.style = 'display:none'
    }
</script>
</html>